(1) Field of the Invention
This invention relates to an IC card having means for protecting erroneous operation due to incomplete electrical contact with an external power supply.
(2) Description of Prior Art
Recently, it has been expected that IC cards are used in many application fields, because of convenience for handling and large memory capacity. The IC cards, which have high security against injustice use by others and can perform various processes by themselves, have been highlighted as a memory medium instead of the magnetic card. The IC cards find many applications for cash cards, personal illness history cards, identification cards, etc.
An IC card is generally made of a plastic card and a semiconductor chip, for example, microcomputer chip and a memory chip sealed in the card. The IC card also has plurality of terminal pads disposed on the surface of the plastic cards. The semiconductor chip for IC card is usually a CMOS microcomputer chip which consumes less operational power.
In use, an IC card is inserted into a card reader and then receives a power supply and control signals from the card reader through the terminal pads. Data reception and transmission are carried out between the IC card and the IC card reader.
A prior art IC card is explained below with reference to FIG. 2. The prior art IC cards comprise a microcomputer 100, disposed on a plastic cards, terminal pads 10, 11, 12, 13 and 14, resistors R1, R2 and R3, and diode D1, D2, D3, D4, D5 and D6, as seen FIG. 2. The terminal pad 10 receives a positive power supply potential (VDD) (for example, +5 volts) supplied from the card reader to the microcomputer 100. The terminal pad 11 receives a resetsignal (RES)from the card reader to the microcomputer 100. The terminal pad 12 receives a clock signal from the card reader. The terminal pad 13 receives a serial data (SIO) from the card reader and transmitts a serial data from the microcomputer 100. The terminal pad 14 receives a reference electric potential (GND) (for example, 0 volts) from the card reader. The protective resistance R1, R2 and R3 are placed between those terminal pad 11, 12 and 13, and the microcomputer. The diodes D1-D6 are connected between the terminal pads 11-13 and the terminal pad 14, forming a protective circuit together with protective resistance R1-R3. The protective circuit configuration is suitable for protecting the gate insulating layers of a CMOS input circuit (see the Japanese Patent Publication No. 48-30189).
The above mentioned IC card operates on an external power source supplied through the terminal pads thereof which is contacted mechanically with the terminal pad of the card reader. If incomplete contact between the terminal pads and the card reader occurs, a potential supplied through the resetsignal input terminal pad 11, the clock signal input terminal pad 12 and the serial signal terminal pad 13, is supplied to the power supply line through the resistors R1, R2 and R3, and the diodes D1 to D6. The potential is dropped by the protective resistors and the diodes, and is unstable because clock signals are input. However the microcomputer formed of CMOS circuits can incompletely operate on such a dropped voltage. Incomplete contact between terminal pads and a card reader may destroy data stored in an EEPROM or an EPROM in the microcomputer.